1. Field of the Invention
This invention relates to a near-field scanning optical microscope in which a probe and a specimen are placed close to each other and are moved relative to each other in a direction nearly parallel to the surface of the specimen so that a region proximate to the surface of the specimen is scanned with the probe, and light derived through the probe is detected by a photodetector to thereby measure optical properties of the specimen.
2. Description of Related Art
A near-field scanning optical microscope is adapted to use a probe whose aperture or radius of curvature at the tip is smaller than the wavelength of light for measuring optical properties of a minute region. Thus, this microscope, which can bring about a resolving power corresponding to the order of the diameter of the probe tip (several tens of nanometers or less), is superior to an optical microscope whose resolving power is restricted by a diffraction limit. In this way, it is expected that such near-field scanning optical microscopes will find increasingly wide applications and uses in the fields of industry and medicine in future.
Thus, in order to realize the near-field scanning optical microscope in which such a high resolving power is obtained, many proposals have been made with respect to measuring methods and structures. For example, methods of detecting optical information are known by those in which illumination light is rendered incident on the back surface of the specimen so that an evanescent wave produced above the front surface (which is hereinafter referred to simply as the surface) thereof Is introduced into the probe for detection; those in which the specimen is irradiated with illumination light from above the surface thereof so that scattered light from the specimen is introduced into the probe with a minute aperture and is detected or reflected light from the probe is detected; and those in which illumination light is emitted from the probe with a minute aperture to detect transmitted light or scattered light from the specimen.
Further, methods for measuring a distance between the specimen and the probe in scanning a region proximate to the surface of the specimen, are known by those in which, in order to utilize intensity attenuation characteristics of the evanescent wave in a perpendicular direction thereof, illumination light is rendered incident on the back surface of the specimen and an evanescent wave produced above the surface of the specimen is detected and measured, and those in which a technique used in an atomic force microscope is utilized to optically detect and measure the displacement of the probe caused by a force exerted between the specimen and the probe.
The near-field scanning optical microscope is constructed so that such technical means are selectively used. In scanning operation, light existing in the vicinity of the surface of the specimen is captured and measured by scanning the specimen while controlling the distance between the specimen and the probe to hold it constant, while maintaining the distance to a predetermined setting value, or while controlling the detected intensity of light to hold it constant. In this way, the surface profile of the specimen or a difference in optical property (absorptance, refractive index, etc.) of the order such that it cannot be measured by an ordinary optical microscope can be imaged on a display such as a TV monitor.
Here, a prior art example of the near-field scanning optical microscope thus constructed is specifically explained with reference to FIG. 1. This example, similar to that disclosed in Japanese Patent Preliminary Publication No. Hei 6-160719 for instance, utilizes the technique used in the atomic force microscope to control a relative distance between the surface of the specimen and the tip of the probe so that it is held constant. A specimen 1 is mounted on a specimen stage 2 constructed with a prism, and light emitted from a light source 3 is totally reflected by the back surface of the specimen 1 so that an evanescent wave is produced above the surface thereof. The specimen stage 2 is designed so that its mounting surface is moved in an X, Y, or Z direction by a scanner 4.
Above the surface of the specimen 1, a probe 5 is placed in such a way that its tip with a minute aperture is brought close to the surface of the specimen 1. In the prior art example, the scanner 4 is moved in the X or Y direction, and thereby a region proximate to the surface of the specimen 1 is scanned with the probe 5. By this scanning operation, the evanescent wave is converted through the probe 5 into a propagation wave, which is collected by an optical collecting system 6 and after passing through a pinhole 7, is detected by a photodetector 8. Detected optical information is introduced through a controller 9 into a computer 10 and is processed there into a signal so that it is displayed as the image of the near-field scanning optical microscope on a monitor 11.
Since the above scanning operation is performed in such a way that the distance between the surface of the specimen 1 and the probe 5 is kept constant, the prior art example utilizes the technique used in the atomic force microscope for this purpose. That is, it is known that if flexibility Is previously imparted to the probe 5, the probe 5 will be deflected by a force exerted between the specimen 1 and the probe 5 in accordance with a change of the distance therebetween. Hence, in the prior art example, light emitted from a light source 12 for probe position control is reflected by the back surface of the probe 5 and the reflected light is detected by a detector 13 for probe position control so that a change of the deflection angle of the probe 5 is taken as that of the reflection angle of the light. In response to its detected signal, the controller 9 moves the scanner 4 in the Z direction and controls it so that the deflection angle becomes constant, thereby maintaining a constant distance between the specimen 1 and the probe 5.
Alternatively, there is a technique of maintaining the distance constant in such a way that a piezoelectric scanner 14 for the probe driven by the controller 9 is provided and, at the same time as the scanning operation, the probe 5 is vibrated in a direction nearly normal to the surface of the specimen 1. This technique, called an AC mode in the atomic force microscope, is such that the constant distance is kept by utilizing the fact that when the probe 5 approaches the specimen 1, the amplitude of vibration is damped by a force exerted between the specimen 1 and the probe 5 or an input vibration is out of phase with the vibration of the probe 5.
The near-field scanning optical microscope of the prior art mentioned above, however, has two problems in practical use. One of these problems refers to the replacement work of the probe 5. Generally, in the microscope In which a probe whose tip has the shape of a minute needle is brought close to the specimen for scanning as in the near-field scanning optical microscope or the atomic force microscope, its resolving power for detection is governed by the probe. Thus, if the tip of the probe wears, or dust or dirt adheres to the tip, due to the specimen, predetermined performance will cease to be obtainable. Furthermore, the tip of the probe, because of its poor strength, may be damaged by errors during operation. Consequently, if such situations are brought about, the probe must be replaced.
The replacement of probes is made by following the procedure that (1) measurement is stopped, (2) a probe is separated from the specimen at a safe distance, (3) a fixing mechanism mounting the probe is removed from the entire device, (4) the probe is removed from the fixing mechanism, (5) a new probe is mounted, (6) the fixing mechanism is attached again to the entire device, (7) the new probe is brought close to the specimen, and (8) the measurement is started. Since, as already mentioned, the probe itself is extremely small, it is not easy to manipulate the probe. Moreover, in the case where the replacement work requiring such labor and time must be done in the midst of the observation of the specimen, the work becomes cumbersome. In particular, if probes are introduced into production lines of a shop, production efficiency will be seriously affected.
The other problem encountered in handling the near-field scanning optical microscope of the prior art is raised when a plurality of probes of different types must be used to measure the same sample. Near-field scanning optical microscopes, as already mentioned, are available in some types, depending on technical means used. Similarly, probes attached to such microscopes are available in many types, such as a probe with an aperture, a metallic probe with no aperture, and a functional probe whose tip is coated with fluorescent pigment. Since these probes have both merits and demerits, there is the need to consider their use in accordance with applications and kinds of specimens. Thus, whenever it is intended to measure a wide object a single microscope as in the case of use in a research institution, probes of different types must be replaced to select the most suitable probe. This impairs work efficiency.
An experimental example that detection sensitivity is governed by a difference in optical property between the specimen and the probe is known by, for example, Kataoka and Endo, "Small Portion Type Near-field Scanning Optical Microscope", J. Optics, Near-field Optical Research Group, First Research Discussion Drafts, pp. 33-39 (June, 1994). As will be understood from this publication, even if one of the probes which are identical in type is used, a specimen will be observed in a condition that a predetermined sensitivity is not obtained, unless a probe suitable for the specimen to be observed is selected and used. This brings about a measurement with degradation of an S/N ratio. In this way, even from this point of view, the most suitable probe must be selected by the replacement of a plurality of probes, but in this case also, it is very disadvantageous to carry out the replacement work stated above.